1. Field of the Invention
The present invention relates to a double-feed detection apparatus and an image forming apparatus. More particularly, the present invention relates to detection of double feed of sheet-like members in an apparatus for conveying the sheet-like members, and also relates to an image forming apparatus for forming images on the sheet-like members.
The term “image forming apparatus” implies an apparatus of the type forming an image on a sheet-like member by employing, e.g., electrophotography. Examples of the image forming apparatus include an electrophotographic copying machine, an electrophotographic printer (such as a laser beam printer or an LED printer), a facsimile, and a word processor.
2. Description of the Related Art
In an image forming apparatus such as a copying machine or a laser beam printer, an image has hitherto been formed on a transfer material (sheet-like member) as follows. First, transfer materials set in a sheet-like member stacking unit, which serves as a paper feed unit, are separated one by one. The separated transfer material is conveyed to an image forming section including various image forming units. In the image forming section, a formed image is transferred onto the transfer material by a transfer unit. Thereafter, the image is fused to the transfer material by a fusing unit, and the transfer material is ejected out of the apparatus. The image is formed on the transfer material through the above-described steps. When the transfer materials set in the paper feed unit are separated and conveyed, there may occur such a state that two or more transfer materials are conveyed to the transfer unit or the fusing unit in an overlapped relation (hereinafter referred to as “double feed”). If the transfer materials in the double-feed state are conveyed, as they are, up to the transfer unit or the fusing unit, the fusing unit being located downstream of the transfer unit, in the image forming section for forming the image, the transfer and the fusing are often not performed under appropriate conditions. In other words, the transfer materials need to be prevented from being conveyed in the double-feed state to the transfer unit or the fusing unit, which is the image forming unit for forming the image on the transfer material. In the following description, the transfer unit or the fusing unit is also called a double-feed prohibition unit.
For that reason, a technique has hitherto been widely practiced in which a double-feed detection mechanism for detecting double feed is disposed on a conveying path extending from a position where the transfer material is separated in the paper feed unit to the transfer unit and, upon detection of the double feed, the transfer materials in the double-feed state are stopped before reaching the double-feed prohibition unit, i.e., the transfer unit or the fusing unit in the image forming section.
In the above-described double-feed detection mechanism, if the transfer materials are double-fed with a mutual deviation in the feed direction (hereinafter referred to as “dragged-in double feed”), there is a possibility of generating erroneous detection when the double-feed detection is performed with respect to only the leading end of the transfer material. For example, the dragged-in double feed is not correctly detected (an erroneous detection indicating no double-feed is resulted) in some cases when the double-feed detection is performed with respect to only the leading end of the transfer material.
In view of such a problem, Japanese Patent Laid-Open No. 2001-063872 proposes a double-feed detection mechanism. More specifically, the double-feed detection mechanism includes a light-emitting unit, e.g., an LED, arranged near a conveying path along which a transfer material is conveyed, and a light-receiving unit, e.g., a photo-transistor. Further, a light quantity received by the light-receiving unit is sampled within a sample range on the transfer material in a predetermined number of samples, and double-feed detection is performed based on each sampled light quantity data. Double feed of the transfer materials is determined based on the results of plural double-feed detections obtained from the sample range.
However, the above-described known double-feed detection mechanism has the following problems.
In the above-cited Japanese Patent Laid-Open No. 2001-063872, when the double feed is detected, the determination as to the double feed is made after the completion of all samplings within the sample range. Therefore, when the sample range is larger than the distance from the double-feed detection mechanism to a particular member, such as a roller or a belt, (in the double-feed prohibition unit), the double feed cannot be determined before the transfer material reaches the particular member, such as the roller or the belt. The particular member, such as the roller or the belt, is a member for forming an image on the transfer material in the image forming apparatus. In other words, such a member is included in, e.g., the transfer unit or the fusing unit in which the transfer materials are apt to jam (namely, a paper jam tends to occur) if the transfer materials are conveyed in the double-feed state. Particularly, the fusing unit is a unit in which a difficulty arises in a process of removing the jammed transfer materials after the occurrence of a jam. Thus, it is often difficult to perform a process for coping with a jam occurred when the transfer materials are conveyed in the double-feed state. If a jam occurs, the transfer materials are wastefully consumed. Further, if the transfer materials are conveyed up to the fusing unit in the double or more feed state, the fusing unit may fail.
If the sample range is widened, a light emission time of the light-emitting unit made of, e.g., an LED is prolonged correspondingly and the light-emitting unit is degraded to a larger extent. Large degradation of the light-emitting unit causes such a risk that the quantity of emitted light is reduced and accuracy of the double-feed detection is reduced. Conversely, when an LED capable of stably emitting light for a long time is used, the apparatus cost is increased because of the necessity of using an expensive LED.